NEW YORK (AP) — Wal-Mart Stores Inc. once again leads Fortune's list of the 500 biggest U.S. companies by revenue, as the world's largest retailer succeeded in posting strong growth despite a challenging economy for its shoppers.

Exxon Mobil Corp. dropped to the second spot, with revenue of $449.9 billion, but was still the most profitable. Energy companies continued to dominate the top of the list, with rival oil and gas producer Chevron Corp. holding steady at No. 3 and refiners Valero Energy Corp. and Phillips 66, spun off from ConocoPhillips last year, joining the top 10.

Fortune Magazine released its annual ranking on Monday.

Moving up two spots to No. 5 is Warren Buffett's Berkshire Hathaway Inc., which owns everything from insurers to railroads to newspaper publishers and is in the process of acquiring Heinz Co. Manufacturing stalwarts General Motors Co., Ford Motor Co. and General Electric Co. slipped in the survey but all remained in the top 10.

Bigger sales of iPads and iPhones helped Apple Inc. jump 11 spots to crack the top 10 for the first time, landing at No. 6. The Cupertino, Calif., company's soaring stock price also made it one of the most valuable companies by market cap last year, though shares have since posted a double-digit decline.

Another technology bellwether wasn't as fortunate. Hewlett-Packard Co. slipped to No. 15 from No. 10 as the Palo Alto, Calif., technology pioneer struggled with a broad consumer shift from PCs to smartphones and tablets and its own accounting missteps.

Also off the top 10 is government mortgage provider Fannie Mae, which dropped four spots to No. 12.

Meanwhile, social media powerhouse Facebook Inc., which last year went public in one of the biggest initial public offerings ever, debuted at 482 on the Fortune 500.